1. Field of the Invention
The present invention relates generally to an optical transmission system for transmitting prescribed information utilizing light. In particular, the present invention relates to an optical isolator which transmits light in one direction without loss, but intercepts the light transmission in the opposite direction.
2. Description of the Prior Art
Nowadays, as the social activities become diverse and complicated, the amount of information communicated by human-to-human, human-to-computer, computer-to-machine, etc., has been gradually increased.
Accordingly, various techniques for transmitting the diverse and large-scaled information accurately, rapidly, and distantly have been steadily developed. As one among such developed techniques, an optical transmission system has been commercialized, and this brings the necessity for optical devices having more diverse functions.
One of such optical devices for use in the optical transmission system is an optical isolator. To protect a light source, for instance, such as laser diodes, the optical isolator passes the light in a forward direction, but intercept the light transmitted in a backward direction.
FIGS. 1 and 2 illustrate the structure of a conventional optical isolator which is disclosed in U.S. Pat. No. 4,548,478.
This conventional optical isolator includes a Faraday rotator 4 for rotating by 45.degree. the light incident through an optical path between first and second optical fibers 1 and 7, and first and second birefringent elements 3 and 5 placed in front and rear of the Faraday rotator 4, respectively.
Each of the first and second birefringent elements 3 and 5 has a tapered surface with a predetermined tapered angle .theta., and the optical axes thereof cross each other with an angle of 45.degree..
The optical isolator also includes first and second collimating lenses 2 and 6 for converting the light emitted from the first and second optical fibers 1 and 7 into parallel light beams.
According to the conventional optical isolator, as shown in FIG. 1, in the event that the light travels in a forward direction, i.e., from the first birefringent element 3 to the second birefringent element 5, the light emitted from the first optical fiber 1 is converted into parallel light beams, by passing through the first collimating lens 2.
The parallel light beams transmitted from the first collimating lens 2 is entered into the first birefringent element 3, and then divided into ordinary rays o and extraordinary rays e. These ordinary and extraordinary rays o and e are rotated by 45.degree. by the Faraday rotator 4.
Thereafter, the ordinary and extraordinary rays o and e, which have been rotated by 45.degree., pass through the second birefringent element 5 to be refracted and converted into parallel light beams. This parallel light beams are condensed through the second collimating lens 6, and then entered into the second optical fiber 7.
Meanwhile, as shown in FIG. 2, in the event that the light travels in a backward direction, i.e., from the second birefringent element 5 to the first birefringent element 3, the parallel light beams transmitted from the second collimating lens 6 are entered into the second birefringent element 5, and then divided into the ordinary rays o and the extraordinary rays e. These ordinary and extraordinary rays o and e are rotated clockwise by the Faraday rotator 4.
At this time, since the optical axes of the first and second birefringent elements 3 and 5 cross each other, the direction of the ordinary rays o make a right angle with that of the extraordinary rays e, and this causes the ordinary and the extraordinary rays o and e incident to the first birefringent element 3 to be reversed from each other.
Accordingly, the reversed light rays o.sub.e and e.sub.o having passed through the first birefringent element 3 cannot become parallel beams, but respectively pass through the first collimating lens 2 with angles predetermined in accordance with the tapered angle .theta. of the first and second birefringent elements 3 and 5. Accordingly, the reversed light rays travel in left and right directions, or upper and lower directions of the first optical fiber 1, and thus cannot be entered into the core of the first optical fiber 1.
As a result, if the light travels in the backward direction of the optical isolator, a great loss of light is produced, and thus the light transmission is intercepted.
However, according to the conventional optical isolator, since the first and second birefringent elements 3 and 5 have only one tapered surface, only one optical signal can be propagated through the optical fibers 1 and 7, resulting in that only one optical signal can be transmitted by one optical isolator.
Accordingly, in the wavelength division multiplexing communication fields utilizing a plurality of optical signals, a plurality of optical isolators should be employed to simultaneously transmit the optical signals, and this causes the size of the optical system and the manufacturing cost thereof to be increased.